Add support for boolean outputs in model runner

Comparison operators produce boolean outputs, which need to be written into client data

Allow subgraph traverser to use main block to look for tensors when serialization handler is missing

Signed-off-by: Jiacheng Liang <jiacheng.liang@arm.com>
Change-Id: I6f9af470185541fa6466b3f7786c48f1555fa6f6